The Wess-Zumino Dark Radiation model (WZDR) [1]} is a simple and natural example of a dark sector with a mass threshold. It contains just two particles - a fermion \(\psi \)  and a scalar \(\phi \) , which interact through a Yukawa coupling \(\phi \psi \psi \) . If we allow for scalar quartic interactions \(\phi ^4\) , this model is efficiently packaged into the simplest known supersymmetric model, namely, the Wess-Zumino model.
Supersymmetry breaking induces a small mass \(m_\phi ^2 \phi ^2\)  for the scalar, which can naturally lie near the  eV  scale. Importantly, the dynamics we discuss are independent of supersymmetry, although this provides natural model-building directions.
